Magnetic recording media generally comprise a magnetic layer coated onto at least one side of a nonmagnetizable substrate. A conventional magnetic recording media, known as particulate magnetic recording media, has a magnetic layer comprised of a magnetic pigment dispersed in a polymeric binder. The magnetic layer may also include other components such as lubricants, abrasives, thermal stabilizers, catalysts, crosslinkers, antioxidants, dispersants, wetting agents, fungicides, bactericides, surfactants, antistatic agents, nonmagnetic pigments, coating aids, and the like.
Some forms of magnetic recording media, such as magnetic recording tape, may also have a backside coating applied to the other side of the nonmagnetizable substrate in order to improve the durability, conductivity, and tracking characteristics of the media. Conventional backside coating include a polymeric binder and other components such as nonmagnetic pigments (e.g., carbon black), lubricants, abrasives, thermal stabilizers, catalysts, crosslinkers, antioxidants, dispersants, wetting agents, fungicides, bactericides, surfactants, antistatic agents, coating aids, and the like.
The polymeric binders of the magnetic layer and the backside coating are commonly derived from curable polymers capable of providing the magnetic recording media with appropriate physical and electromagnetic properties when cured. To prepare such media, the components of the magnetic layer or the backside coating are dispersed in a suitable solvent and the resulting dispersion coated onto the nonmagnetizable substrate, dried, optionally calendered, and cured.
Polymeric binders used in the manufacture of magnetic recording media are most commonly blends of a hard resin component (i.e., a polymer with a relatively high glass transition temperature and modulus), and a soft resin component, (i.e., a polymer with relatively low glass transition temperature and modulus). For example, JP 01-073523A and U.S. Pat. No. 4,837,082 disclose that the soft and hard resin components of a polymeric binder must be selected properly in order to maintain flexibility whilst ensuring that the resulting cross-linked polymer matrix has the required tensile strength and modulus.
Hard Resin Component
Copolymers based on vinyl chloride or vinylidene chloride are widely used as the hard component of choice, particularly when the soft resin component is a polyurethane. Such copolymers are generally highly miscible in and compatible with polyurethanes, and possess a relatively high glass transition temperatures, moduli, and hardness. For example, Japanese Publication No. 61-26132 describes the use of vinyl chloride/vinyl acetate/vinyl alcohol copolymers as a polymeric binder component in magnetic recording media.
A drawback associated with the use of vinyl chloride or vinylidene chloride copolymers as the hard resin component in a polymeric binder is the tendency of such copolymers to degrade over time, resulting in the release of highly corrosive gaseous HCl and a change in the properties of the media. Accordingly, some investigators have described various nonhalogenated vinyl copolymers for use in magnetic recording media. See, e.g., U.S. Pat. Nos. 5,501,903, 5,098,783, 4,876,149, and 4,837,082, and Japanese Publication Nos. SHO 62-30162, SHO 54-84708, SHO 54-46519, and SHO 54-46518.
Soft Resin Component
Polyurethane polymers are widely used as the soft component.
Dispersants
Magnetic pigments tend to agglomerate and can be difficult to initially disperse in the polymeric binder. In addition, magnetic pigments have a propensity to settle within polymeric binders over time. In order to facilitate dispersion of the pigments and stabilize the resulting dispersion, low molecular weight wetting agents, or dispersants, are often employed. Greater amounts of wetting agent or dispersant may be required when higher pigment loadings are employed (i.e., greater amounts by weight of magnetic pigment) and/or smaller pigment particles are employed (i.e., increased surface area). This is not always desirable. Dispersants may plasticize a binder systems and can decrease the modulus of the system. In addition, excess dispersant may exude from a cured binder system over time, leading to changes in the properties of the media and contamination of a recording head.
Self-Wetting Polymers
In an effort to reduce or eliminate the need for dispersants, polymeric binders formed from "self-wetting" polymers have been developed. "Self-wetting" polymers are polymers having pendant dispersing groups effective for facilitating dispersion of a magnetic pigment within the polymer. Representative examples of dispersing groups include quaternary ammonium groups, amines, heterocyclic moieties, salts or acids based on sulfate, salts or acids based on sulfonate, salts or acids based on phosphate, salts or acids based on phosphonate, and salts or acids based on carboxyl, as well as mixtures thereof.
Use of self-wetting polymers generally reduces the amount of dispersant and/or wetting agent required to disperse the magnetic and/or nonmagnetic pigments in the polymeric binder.
SELF-WETTING HARD RESINS
Self-wetting vinyl chloride copolymers are well known and have been described in several United States Patents, including U.S. Pat. Nos. 5,139,892, 5,126,202, 5,098,783, 5,064,730, 5,028,676, 5,008,357, 4,861,683, 4,784,913, 4,770,941, and 4,244,987.
SELF-WETTING SOFT RESINS
Self-wetting polyurethanes have been proposed, including polyurethanes having a phosphorous-based wetting functionality, referenced herein as "P functional" polyurethanes. The phosphorus functionality can be pendant to the main polymer chain or an integral part of the chain. Self-wetting P-functional polyurethanes are disclosed in JP 01-106324A, JP 02-053219A, and U.S. Pat. Nos. 5,501,903, 5,491,029, 4,612,244 and 4,842,942.
Several patents disclose polyurethanes with both phosphorous-based and hydroxy functionalities at sites other than at chain ends. For example, JP 02-012613A describes P functional polyurethanes with tertiary hydroxy functionality at positions other than chain ends.
SELF-WETTING POLYMERIC BINDERS
There are several types of polymeric binder blends containing self-wetting polymers. These can be divided into three groups: (a) self-wetting polyurethanes with "non-wetting" hard resins, (b) "non-wetting" polyurethanes with self-wetting hard resins, and (c) polyurethanes and hard resins which both contain polar wetting functionalities.
Examples from group (a) combine self-wetting polyurethanes with halogenated and nonhalogenated vinyl polymer hard resins. JP 02-012613 discloses self-wetting polyurethanes blended with vinyl chloride/vinyl acetate, vinyl chloride/vinyl propionate, thermoplastic polyurethanes, cellulose polymers, thermoplastic polyesters, epoxy resins, and phenolic resins. U.S. Pat. No. 5,501,903 discloses self-wetting phosphonated polyurethanes blended with nonhalogenated vinyl polymers. Other examples of such blended polymeric binders can be found in JP 3224130A, JP 3224129A, JP 3224128A and U.S. Pat. No. 4,529,661.
Examples from group (b) combine polyurethanes with self-wetting vinyl polymer hard resins. U.S. Pat. No. 5,151,330 discloses a polymeric binder including a polyurethane resin containing polycaprolactone units and a self-wetting vinyl chloride resin containing various polar groups, such as phosphonic and phosphoric acids and salts. Other examples are described in JP 3041618 and U.S. Pat. No. 5,096,774.
Examples from group (c) combine a polyurethane and a hard resin component wherein both contain polar groups effective for promoting wetting and dispersion of a pigment within the polymeric binder. The polar group of choice is generally a sulfur-based or phosphorous-based acid or salt. JP 3203020 discloses a polymeric binder including a combination of a vinyl chloride, cellulose derivative or polyester resin and a polyurethane or polyester resin, with each of the resins containing a polar group selected from SO.sub.3 M, COOM or PO(OM).sub.2, wherein M is hydrogen, an alkali metal, or ammonium. U.S. Pat. No. 5,491,029 discloses a polymeric binder including a combination of a phosphonated polyurethane and a vinyl chloride copolymer having pendant ammonium functionalities. Other patents which disclose such polymeric binders include JP 3141018, and U.S. Pat. Nos. 4,830,923 and 4,784,914.
While polymeric binder systems including a self-wetting hard and/or soft resin component provide a significant advance over previous polymeric binder systems, a continuing need exists for improved self-wetting polymers capable of forming a polymeric binder system having improved performance characteristics, such as an improved modulus, Tg, and smoothness.